1. Field of the Invention
This invention relates to an image display device for reproducing and displaying an electrical image information as computing output or reading output from a computer or an image reading device, an image information accumulated and memoried in a magnetic tape or microfilm as a soft copy in the form of a visible image, or an image display device to be attached as an image monitor, etc. to a machine related to image formation such as copying machine, office automation apparatus, etc.
More particularly, the present invention pertains to improvement of an image bearing member for a device which effects image display or image formation by forming a toner image as the display image corresponding to inputs of various information signals as mentioned above on the surface of an adequate image bearing member.
2. Description of the Prior Art
As the image display device known in the art, for example, CRT display devices and liquid crystal devices have generally been employed widely. These display devices, however, are not necessarily satisfactory in resolution, size of the display picture face or visibleness. For example, in view of the recent progress of office automation machines, the display picture face required for monitors for word processors, microfilm searching and optical disc memory is a highly precise stationary image, and therefore CRT or liquid crystal cannot be stated to be adequate due to flicker or dependence on vidual angle.
On the other hand, other than the aforesaid CRT display or liquid crystal display, for example, U.S. Pat. application Ser. No. 445,070 (Tamura et al, filed on Nov. 29, 1982) disclosed a method, in which a photoconductive layer is used as the image bearing member and a toner image is formed simultaneously with image exposure to make the toner image a display. Japanese Utility Model Laid-open Publication No. 55061/1982 discloses a method in which an electrostatic image is formed by scanning a pin electrode on a dielectric belt of image forming member and the toner image is obtained corresponding to the electrostatic image and exhibited as a display. Alternatively, there is also a method in which a toner of magnetic powder or magnetic gas is visualized and displayed on the surface of an image bearing member by a readily magnetizable stylus corresponding to the magnetizing signals.
In the display obtained according to such image forming methods, an image quality comparable to printing can be obtained. Particularly, since it is a stationary image display, as compared with the dynamic image display (the system in which image elements are displayed under lighted state by repeating lighting and non-lighting through utilization of the after image effect of eyes) such as CRT display or liquid crystal display, it can be made into a display which can be very easily viewed without fatigue of eyes.
Whereas, some problems are involved in a device of the system wherein the toner image obtained by the image forming method as described above is displayed. That is, because the same image member is used repeatedly, the toner or the colorant used therefor are gradually attached on the image display surface, whereby the non-image portion will markedly be stained. More specifically, displaying according to this system comprises the steps of forming a latent image on an image bearing member with the information signal in a time series signal such as light (e.g. laser beam), or voltage or current, and subsequently visualizing the latent image by developing with a toner. When the display is no longer necessary, via an extinguishing step, an image pattern is formed again according to the same steps on the same image bearing member surface, thus effecting display repeatedly. Accordingly, staining of the surface of the image bearing member damaged markedly the image quality, which was an obstacle in practical application of the display device according to this system. As another device according to a system employing similar steps for obtaining the image, there is a device in which a toner image is formed on an intermediate master such as those employed in electrophotographic copying, electrostatic printing, magnetic printing, etc., and transferred onto a plain paper to obtain a print image. The intermediate master to be used in these print devices, which is also used repeatedly, poses no problem even if the surface itself may be stained, provided that it has no effect on the transferred image. Thus, the performance required for such an intermediate master is essentially different from that required for the image bearing member to be used in the display device as described above. As still another problem in this kind of display device, the image density or sharpness will be changed depending on the use environment to be changed greatly image quality More specifically, since displaying according to this system comprises the cycle of toner image formation-displaying-extinction as disclosed above, the change in image quality depending on the environment has been an obstacle in practical application of the display device according to this system. As another device according to a system employing similar steps for obtaining the image, there is a device in which a toner image is formed on an intermediate master such as those employed in electrophotographic copying, electrostatic printing, magnetic printing, etc., and transferred onto a plain paper to obtain a print image as aforementioned. In these print devices, various controlling mechanisms for forming adequate image, such as means for toner supplement or heating for temperature maintenance, and further the transferred image can be finished uniformly as the print image by the fixing step. In contrast, in this kind of display device, there are limits to these controlling means and besides no fixing step can be conducted. Therefore, the image bearing member to be used in this kind of display device is required to have characteristics with less dependence on the environment as compared with the intermediate master in a printer device. As still another problem, during repetitive use of the same image bearing member, a large number of flaws are generated, whereby staining is increased by image defects or the toner embedded in the flaws to result in marked lowering in image quality. That is, since displaying according to this system comprises the cycle of toner image formation-displaying-extinction as disclosed above, staining of the display surface of the image bearing member by flaws or embedding of the toner in the flaws will damage markedly the image quality, and it was a great obstacle in practical application of the display device of this system.